A steam turbine is used to drive a machine or the like and includes a turbine main body having a rotor which is rotatably supported. The rotor is rotationally driven by supplying steam serving as a working fluid to the turbine main body. Steam supplied to the turbine main body or steam extracted from the turbine main body flows through the steam flow path of the steam turbine. An adjustment valve is provided in the steam flow path. It is possible to adjust a flow rate of the steam supplied to the turbine main body by adjusting an opening degree of the adjustment valve.
The adjustment valve is driven by an adjustment valve drive device. For example, as the adjustment valve drive device, there is a device having an electric motor serving as a drive source and a linear motion mechanism such as a ball screw which converts a rotational motion of the electric motor to a linear motion of the adjustment valve.
For example, the linear motion mechanism is disclosed in PTL 1. In the linear motion mechanism disclosed in PTL 1, a screw shaft having a screw groove on an outer peripheral surface and a nut having a screw groove on an inner peripheral surface are combined and a transfer path is formed by both screw grooves facing each other. In the linear motion mechanism, since a plurality of balls roll on the transfer path, a rotational motion and a linear motion are converted to each other by a small friction force.